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Hawksbill Turtles in Seagrass Beds

Carlos E. Diez1, Ximena Vlez-Zuazo2 & Robert P. van Dam2

1Departamento de Recursos Naturales y Ambientales. P.O. Box 906600, San Juan, PR 00906-6000

(E-mail: cediez@caribe.net) 2Chelonia Inc. P.O. Box 9020708, San Juan, PR 00902-0708

(E-mail: xvz_1974@yahoo.com & rpvandam@yahoo.com)

Coral reefs are often reported as prime habitat ofthe hawksbill turtle (Eretmochelys imbricata; Carr &Stancyk 1975; Len & Mota 1996). Several authors(Van Dam & Diez 1996 & 1998; Witzell 1983) havedescribed sparse hard-bottom communities, cliff-wallhabitats with soft corals and invertebrates as importantbenthic developmental habitat for hawksbills. Otherstudies (Andares & Uchida 1994; Limpus 1992; VanDam & Diez 1997b; Vicente & Carballeira 1991) alsoindicate the hawksbills preference for coral reefecosystems. Such habitat characterizations have led toarea estimates of potential habitat for hawksbill turtles

using the distribution of coral reefs (Buitrago & Guada2002; Meylan et al. 1997; Prieto et al. 2001). Someresearchers, however, have also reported occasionalhawksbill sightings in other habitats, such as lagoonswith seagrass areas in the Bahamas (Bjorndal & Bolten1988).

A similar pattern has often been observed with greenturtles (Chelonia mydas). For example, aggregationsof this species, which are usually associated withseagrass beds as their main feeding grounds, are alsoreported on coral reefs (Balazs 1979; Hirth 1992). As

with green turtles, it is probable that hawksbill turtlesfrequently use habitats other than coral reefs for feedingand development. Here we report on the results of apreliminary in-water survey for marine turtles conductedon June 2003, along the southeast coast of theDominican Republic, where we encountered asignificant aggregation of juvenile hawksbill turtles in ashallow seagrass habitat.

Manglar Study Site is located at Saona Island onthe southeast coast of the Dominican Republic (fig. 1).This site is characterized by seagrass beds protectedby a reef breaker forming a lagoon, where water depth

ranges from 0.5 to 2 meters. The dominant seagrassspecies are turtle grass (Thalassia testudinum) andmanatee grass (Syringodium filiforme). Severalspecies of algae (i.e.Halimeda incrasssata, Penicillusdumetosus and Udotea flabellum), and small coloniesof corals (i.e. Monastrea sp.and Diploria sp.) werealso found at scattered locations throughout the area(Vega et al. 1997, pers.obs.). The chicken-liver sponge(Chondrilla nucula) was also found encrusted amongthe turtle grass.

Visual turtle surveys at the Manglar study site wereconducted with four observers using a small boat andslowly navigating parallel to the shore. On someoccasions we made attempts to catch sighted turtles.This was mostly accomplished by following a sightedanimal until it tired and then by diving onto it from thebow of the boat (following Ehrhart & Ogren 1999).The survey transects start and end points, and turtlecapture locations were recorded using a GPS receiver.

The relative sighting frequency (RSF) of sea turtlesat the Manglar site was evaluated by dividing the numberof turtles captured or sighted by the time spent

surveying, with surveys typically lasting one hour for atotal of three sessions. One session was conductedduring morning hours and two sessions in the afternoon,for two days. The estimated size of all turtles wasrecorded together with species. All turtles handled weremeasured, weighed and marked on both front flipperswith 681c style inconel tags. Individuals were alsochecked for the presence of internal passive integratedtransponder (PIT) tags.

A total of 23 hawksbills and four green turtles weresighted during the boat surveys totaling three hours.

Nine of the 27 turtles were captured. The resultingmean RSF for Manglar is 7.7 hawksbills per hour. Thesize range of the hawksbills captured was 29 to 41 cmcurved carapace length (nuchal notch to posteriormosttip; CCLn-t; n=8), and sighted turtles were all estimatedto be from 25 to 50 cm carapace length. Four greenturtles were seen and one captured (measuring 33.3cm CCLn-t). All green turtles observed were thoughtto be approximately 30 cm CCL.

Although our survey at Saona Island was far fromcomplete, we note that a significant number of juvenileturtles are present, and specifically that hawksbill turtles

inhabit this seagrass community. Whereas other reportsof sporadic hawksbills residing in seagrass habitats exist,we believe the Manglar site is one of the firstdocumented seagrass communities demonstrating alarge abundance of hawksbill turtles. The high RSF ofhawksbills recorded compares with study sites ofimportant aggregations of this species. For example, acatch per unit effort of 3.4 and 4.7 hawksbills per hourhave been recorded at Mona-Monito Island and Jaragua,Dominican Republic, respectively (Diez & Van Dam

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Figure 1. Map of surveyed area at Saona Island, Parque Nacional del Este, Dominican Republic.

2001; Len & Diez 1999). We regard that this area is adevelopmental habitat, from the size range of turtlesobserved and captured in the area. Even though we didnot conduct stomach content analyses, we speculatethat these hawksbills are feeding on sponges such aschicken liver sponge (C. nucula), which is a commonprey item for Caribbean juvenile hawksbills elsewhere

(Andares & Uchida 1994; Leon & Bjorndal 2002;Meylan 1988; Van Dam & Diez 1997a).In conclusion, juvenile hawksbill turtles living in

habitats other than coral reefs may be more commonthan suspected. Further examination of other benthiccommunities for the presence of hawksbill turtles maywiden the scope of characteristic habitats recognizedfor this species in the Caribbean. We caution that notonly the most characteristic habitat type should beexamined in any determination of the conservation statusof sea turtle species.

Acknowledgements: To all the staff of Ecoparque, Inc. andthe Comandancia de la Marina de Guerra de Bayahibe, Rep.Dominicana for the logistical and human support providedin the field. We would like to especially thank Kelvin Guerreroand our field assistants: Pedro Javier Santana, Flix Soto,Bienvenido, Chichi, El Negro Milady and El Toro. Also, wewant to thanks Matilde Mota from the Ministerio del MedioAmbiente de la Repblica Dominicana for arranging the

research permit and Karen Bjorndal for kindly reviewing thismanuscript. Project support came from Japan BekkoAssociation, Ecoparque Inc., Chelonia, Inc., andDepartamento de Recursos Naturales y Ambientales dePuerto Rico.

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